Flowmeters measure the rate of flow of a fluid in a pipe or other pathway. The fluid may be, for example, a gas or a liquid, and may be compressible or incompressible. One type of flowmeter is a vortex flowmeter, which measures parameters including, for example, flow rate based on the principle of vortex shedding. Vortex shedding refers to a natural process in which a fluid passing a bluff body (sometimes referred to as a shedder) causes a boundary layer of slowly moving fluid to be formed along the surface of the bluff body. A low pressure area is created behind the bluff body and causes the boundary layer to roll up, which generates vortices in succession on opposite sides of the bluff body. The vortices induce pressure variations that may be sensed by a pressure sensor. The vortex-shedding pressure variations have a frequency that is related to the flow rate. Accordingly, by measuring the frequency of the pressure variations, the flow rate may be determined.
Vortex flowmeters provide vortex frequency data that can be used in conjunction with flow calibration factors to determine the velocity and volumetric flow rate of the fluid passing through the meter. With fluid density values, the mass flow rate can also be computed. These measurements, and others, can be transmitted to a control room or other receiver over a communication line, such as, for example, a standard two-wire 4-20 milliamp (“mA”) transmission line.
Vortex meters encounter problems if the fluid flow rate is too low because the fluid may have insufficient velocity to result in periodic vortex formation at regular intervals. One solution to this problem is to use a constriction to increase the velocity of the fluid as it flows by the bluff body relative to the fluid upstream and downstream of the bluff body. For example, the inner surfaces of the flowtube can be tapered to reduce the diameter of the cross-sectional flow area at the position of the bluff body. This approach is explained in greater detail in commonly owned U.S. Pat. No. 7,533,579, the contents of which are hereby incorporated by reference. Although this approach can improve the ability of a vortex meter to operate in low flow conditions, it is still possible for the fluid velocity to drop too low for the meter to take accurate measurements of the fluid flow. The problem can be especially troublesome with very large vortex meters (e.g., those having diameters in the range of about 12 inches or more). For example, a relatively large amount of fluid can flow through the meter even in low velocity conditions because of the large cross-sectional flow area.
The present inventors have developed systems and methods, described in detail below, that improve the ability operate a vortex flowmeter under low flow conditions.